The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical diagnostic imaging and will be described with reference thereto. It is to be appreciated, however, that the invention may find further application in quality control inspections, spectroscopy, and the like.
Magnetic resonance imaging (MRI) machines operate by applying a main magnetic field through an examination region to align the nuclei of a subject. This main magnetic field, typically denoted B0, is horizontal in some MRI systems and vertically oriented in others.
In both horizontal and vertically oriented MRI systems, a magnetic resonance is excited in the aligned nuclei by an orthogonal RF field B1 causing them to emit RF resonance signals. This resonance is detected by a radio frequency (RF) coil that is tuned to the resonance frequency. The signals received by the coil depict the three dimensional spatial distribution and other characteristics of the resonating nuclei. Of the many RF coil configurations, birdcage type coils are commonly used for head and whole body imaging.
Birdcage coils are used widely for many applications, especially in horizontal B0 MRI systems where it is convenient to align the axis of the coil with the B0 field and there is good B1 field uniformity over large fields of view. For most applications, birdcage coils are used on a quadrature mode. Typically, these quadrature birdcage coils are disposed axially in horizontal B0 MRI machines such that the coil has orthogonal B1 modes perpendicular in the B0 field. In other words, the coil axis is parallel to the B0 main magnetic field axis. Accordingly, the B1 fields most useful in such a coil are those, preferably, orthogonally disposed to the B0 axis.
Quadrature birdcage type coils can also be used in a vertically oriented B0 MRI machine. However, when the birdcage coil is oriented with its axis horizontal, the quadrature capability of the coil fails or gets degraded as some of the B1 components in the legs or rungs of the coil are parallel to the vertical B0 direction. Accordingly, other types of quadrature coils are typically used in vertical B0 machines, and in other cases where the B0 field is not parallel with the axis of the coil.
The present invention contemplates a new, improved birdcage coil supporting three axis modes providing quadrature reception irrespective of B0 field direction. The present invention thus, overcomes the above difficulties and others.
In accordance with the present invention, a magnetic resonance method in which a magnetic field is generated along an axis through an examination region is shown. Radio frequency signals are transmitted into the examination region to induce magnetic resonance in nuclei of a desired object. The induced magnetic resonance is received by a birdcage or volume coil and is processed into an image representation. The method includes extracting an end ring resonant mode signal from the volume coil.
In accordance with another aspect of the present invention, the extracting step includes inductively coupling a loop to the end ring resonant mode signal.
In accordance with another aspect of the present invention, a magnetic resonance apparatus includes a main magnetic field generator for providing a main magnetic field along an axis. A magnetic resonance exciter excites nuclei of an object to resonate, which generates magnetic resonance signals. A volume coil is also provided, which includes a pair of end rings separated along a coil axis. The end rings are electrically interconnected by a plurality of rungs disposed about a periphery of the end rings. A conductive element, also provided, inductively couples to the end rings.
In accordance with another aspect of the present invention, a volume radio frequency coil for use in a magnetic resonance apparatus includes a main magnetic field generator for providing a magnetic field along a axis. A radio frequency transmitter and a radio frequency receiver are also provided at least one of which is connected with the RF coil. The RF coil includes a pair of conductive end rings disposed concentrically about a coil axis in parallel planes. A plurality of conductive rungs disposed about a periphery of the end rings provide electrical interconnection therebetween. An electrically conductive loop is also provided to inductive couple to the pair of end rings.
One advantage of the present invention resides in implementation of a conventional sinusoidal or cosinusoidal resonant mode and an end-ring resonant mode at the same frequency to provide three axis capability.
Another advantage of the present invention resides in the provision of quadrature reception and/or excitation anywhere in the imaging region, regardless of B0 orientation.
Yet another advantage of the present invention resides in a full volume RF coil offering good B1 uniformity regardless of B0 alignment.
Still another advantage of the present invention resides in the ability to extract three orthogonal modes from a volume RF coil.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiment.